This invention relates to apparatus for the treatment of liquid media by gasification. One application of such apparatus is to the gasification of liquid media which contain organic matter degradable by the action of gases thereon, for example the aeration or oxygenation of sewage or other waste water.
British Pat. No. 1,482,191 discloses a device for the gasification of liquids which comprises a hollow ring member having a central liquid passageway and a gas inlet. A portion of the surface facing the passageway is apertured so that in operation the gas passes from the hollow into liquid in the passageway. The cross-section of the liquid passageway progressively increases away from the apertured area.
In operation that device is immersed in a liquid so that the liquid passageway has its longitudinal axis substantially vertical. The gas forms bubbles on passing through the apertures into the liquid in the passageway, the bubbles rising through the liquid to induce a flow of liquid upwardly through the passageway. The induced liquid flow passing the apertures exerts a shearing effect on the gas bubbles as they leave the ring member thereby producing smaller gas bubbles than in prior art devices and in consequence increasing the overall gas/liquid interface area for a given volume of gas, and thus increasing the rate of gas-to-liquid transfer.
Such devices have been found to be highly effective in practical use, particularly for aeration of waste water containing aerobically degradable organic matter, for example sewage. Such devices achieve a high rate of gas transfer into solution with low energy requirements and are also highly effective in setting up and maintaining a strong circulation pattern in the body of liquid being treated. This latter is of importance when the liquid medium includes a substantial proportion of suspended solids prone to settle out in a potentially anaerobic mass in any region of the medium not subject to a good circulation current.
Attention is further directed to my U.S. Patent Application Ser. No. 25460, now U.S. Pat. No. 4,198,359, of which is incorporated herein by reference, and which discloses various operating parameters that require consideration when constructing a waste water treatment plant and suspending such gasification devices therein, in particular certain dimensions and the positioning of the devices in relation to the treatment vessel for useful results.
British registered Design No. 987,313 shows a particular form of the device that has proved effective in practical use. A practical form is of plastics material about 40 centimeters tall and 40 centimeters in diameter, and in use is suspended by chains in a treatment vessel. The device is weighted, suitably by including concrete (e.g. 30 to 40 kilograms) introduced through the gas inlet to set in the lower portion of the hollow enclosure, to achieve an average density for the device in air greater than that of water and a reasonably low centre of gravity. In that form the device is suspended by individual chains directly shackled to diametrically opposite points of a steel belt surrounding the waist of the device.
Another form of device is disclosed in British Patent Application Ser. No. 7,901,809 which discloses "a device for the gasification of liquids which comprises a hollow annular member having a gas inlet into said hollow and an outer surface which bounds a central liquid flow passageway extending between a lower inlet and an upper outlet, the passageway comprising a lower convergent portion, an intermediate throat portion, and an upper divergent portion, said convergent portion having no apertures therein communicating with said hollow, said throat portion having a row of first apertures extending around a generally horizontal circumference thereof, and said divergent portion having a row of second apertures extending around a generally horizontal circumference thereof, said second apertures being no more than half the diameter of said first apertures". In one embodiment there may be a single row of first apertures of 6 millimeter diameter and a single row of second apertures of 3 millimeter diameter.
As stated above these devices can achieve a high rate of gas transfer and maintain a strong circulation pattern in the body of liquid. It is now found that the strength of each said pattern both in itself and in relation to the position of other devices present and the vessel walls exert considerable forces on the devices. Because the suspension is by chains any such forces which are not directly balanced by chain tension tend to cause various movements of the suspended device as well as fretting and tugging at the shackles. These movements can include bodily movements off station, bodily swinging movement and angular oscillations about each of three manually perpendicular axes. Under certain conditions some of these movements are susceptible to reinforcement by their own occurrence and this can lead to disruption of the desired circulation patterns and to devices becoming entangled or twisted in their suspension chains and thus out of their proper level.
It would be possible to some extent to increase the weight of each device to damp the oscillation but this would have the disadvantages of inconvenience in handling and to greater fretting forces on the suspension points without solving the basic problem. It would also be possible to provide a three or more chain suspension, but this would still be subject to fretting and instabilities and moreover it would be difficult and inconvenient to suspend the device level in itself and level with the other devices, particularly in virtually opaque waste water.
Further disadvantages of current suspension arrangements are inconvenience and inflexibility. It is found that the placement of these devices is important for optimum effectiveness, both as to depth of immersion and to horizontal spacing between devices and from the walls of a waste water treatment vessel. Moreover gasification devices in waste water are liable to blockage or obstruction of the gas introduction apertures. It is therefore necessary from time to time to lift the devices out of the waste water for cleaning or replacement. It is then important to restore the cleaned device, or dispose the replacement device, to the original correct location as to depth and spacing. This exact, repeatable, placement of devices in a desired location in opaque waste water poses a problem with current suspension arrangements.